U.S. patent number 3,812,559 [Application Number 05/216,539] was granted by the patent office on 1974-05-28 for methods of producing field ionizer and field emission cathode structures.
This patent grant is currently assigned to Stanford Research Institute. Invention is credited to Louis N. Heynick, Charles A. Spindt.
United States Patent |
3,812,559 |
Spindt , et al. |
May 28, 1974 |
METHODS OF PRODUCING FIELD IONIZER AND FIELD EMISSION CATHODE
STRUCTURES
Abstract
Field-forming devices primarily useful as field ionizers and
field emission cathodes and having as a basic element an array of
closely spaced cones with sharp points supported on a substrate (in
the most usual case conductive or semiconductive) are disclosed.
Preferably, the field-forming structure is completed by a
screen-like structure, e.g. as fine mesh screen, insulatively
supported above the points with the center of apertures in the
screen substantially aligned with the longitudinal axis of
corresponding cones. A novel method of forming such structures
includes placing a screen with a mesh corresponding to the desired
number and packing density of sharp conical points in close
proximity to, or in contact with, the substrate and projecting
material through the screen onto the substrate whereby sharp cones
of the material are formed on the substrates.
Inventors: |
Spindt; Charles A. (Menlo Park,
CA), Heynick; Louis N. (Palo Alto, CA) |
Assignee: |
Stanford Research Institute
(Menlo Park, CA)
|
Family
ID: |
26732772 |
Appl.
No.: |
05/216,539 |
Filed: |
January 10, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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54222 |
Jul 13, 1970 |
3665241 |
|
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|
Current U.S.
Class: |
445/52; 445/50;
445/24 |
Current CPC
Class: |
H01J
49/16 (20130101); B82Y 10/00 (20130101); H01T
23/00 (20130101); H01J 9/025 (20130101) |
Current International
Class: |
H01J
49/16 (20060101); H01J 49/10 (20060101); H01T
23/00 (20060101); H01J 9/02 (20060101); H01j
009/02 () |
Field of
Search: |
;29/25.17,25.18
;313/309,351 ;117/210,212 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
C A. Spindt, "A Thin-Film Field-Emission Cathode," J. of Applied
Physics, Vol. 39, No. 7, June 1968, pp. 3,504-3,505..
|
Primary Examiner: Lake; Roy
Assistant Examiner: Davie; J. W.
Attorney, Agent or Firm: Faubion; Urban H.
Parent Case Text
This is a division of application Ser. No. 54,222, filed July 13,
1970, now Patent No. 3,665,241.
Claims
We claim:
1. The method of producing an array of conical needle-like points
on a surface comprising the steps of:
a. positioning a plate-like screen member adjacent to said surface,
said plate-like member having apertures therethrough corresponding
in number and location to the desired needle-like points;
b. depositing needle-forming material substantially perpendicular
to the plane of said plate-like screen member from a single
deposition source which is broad enough simultaneously to close the
said apertures in said plate-like member and deposit material on
said surface within said apertures, thereby forming conical
needle-like points within said apertures; and
c. removing said plate-like screen member whereby a bare point
array of conical needle-like points is left on the said
surface.
2. The method of producing a field-forming device including the
steps defined in claim 1 and the additional step of subsequently
placing a plate-like counter-electrode having apertures
therethrough corresponding in number and location to the
needle-like points on said surface in insulated spaced relation
relative to said surface and points on the said surface.
3. The method of producing an array of conical needle-like points
on a surface comprising the steps of:
a. positioning a plate-like screen member adjacent the said
surface, said plate-like member having apertures therethrough
corresponding in number and location to the desired needle-like
points;
b. depositing needle-forming material substantially perpendicular
to the plane of said plate-like screen member from a single
deposition source which is broad enough simultaneously to close the
said apertures in said plate-like member and deposit material on
said surface within said apertures, thereby forming conical
needle-like points within said apertures; and
c. removing all material deposited on said plate-like screen member
whereby said screen member can be used as an electrode of said
field-producing structure.
4. The method of producing a field-forming device which includes an
array of conical needle-like points on a surface comprising the
steps of:
a. positioning a plate-like screen member adjacent to said surface,
said plate-like member having apertures therethrough corresponding
in number and location to the desired needle-like points;
b. depositing needle-forming material substantially perpendicular
to the plane of said plate-like screen member whereby material is
deposited on said second electrode within said apertures, thereby
forming conical needle-like points within said apertures;
c. depositing a masking material at a shallow grazing angle on said
plate-like screen member at the same time the said needle-forming
material is deposited, thereby to provide a lip or mask of
diminishing size around the rim of each aperture therein;
d. removing said plate-like screen member whereby a bare point
array of conical needle-like points is left on the said
surface;
e. depositing an insulating material on a plate-like
counter-electrode having apertures therethrough corresponding in
number and location to the needle-like points on said surface;
and
f. positioning said plate-like counterelectrode adjacent to said
surface whereby said insulation on said counterelectrode provides
separation and insulation from said surface and also provides
registration of said points on said surface and apertures in said
counterelectrode.
5. The method of producing a field-forming device which includes an
array of conical needle-like points on a surface comprising the
steps of:
a. positioning a plate-like screen member adjacent to said surface,
said plate-like member having apertures therethrough corresponding
in number and location to the desired needle-like points;
b. depositing needle-forming material substantially perpendicular
to the plane of said plate-like screen member whereby material is
deposited on said second electrode within said apertures, thereby
forming conical needle-like points within said apertures;
c. depositing a masking material at a shallow grazing angle on said
plate-like screen member at the same time the said needle-forming
material is deposited, thereby to provide a lip or mask of
diminishing size around the rim of each aperture therein;
d. removing said plate-like screen member whereby a bare point
array of conical needle-like points is left on the said surface;
and
e. placing a plate-like counterelectrode having apertures
therethrough corresponding in number and location to the
needle-like points on said surface in insulated spaced relation
relative to said surface and the points on the said surface.
6. The method of producing an array of conical needle-like points
on a surface comprising the steps of:
a. positioning a plate-like screen member adjacent to said surface,
said plate-like member having apertures therethrough corresponding
in number and location to the desired needle-like points;
b. depositing a masking material at a shallow grazing angle on said
plate-like screen member to provide a lip of diminishing size
around the rim of each aperture thereon;
c. depositing needle-forming material substantially perpendicular
to the plane of said plate-like screen member whereby material is
deposited on said surface within said apertures, thereby forming
protrusions thereon;
d. simultaneously depositing a masking material at a shallow
grazing angle on said plate-like screen member and depositing
needle-forming material substantially perpendicular to the plane of
the said plate-like screen member, whereby needle-forming material
is deposited on said surface within said apertures at the same time
the said apertures are closed by said masking material, thereby
forming conical needle-like points within said apertures; and
e. removing said plate-like screen member whereby a bare point
array of conical needle-like points is left on the said
surface.
7. The method of producing an array of conical needle-like points
on a surface comprising the steps of:
a. positioning a plate-like screen member adjacent to said surface,
said plate-like member having apertures therethrough corresponding
in number and location to the desired needle-like points;
b. depositing a masking material at a shallow grazing angle on said
plate-like screen member to provide a lip of diminishing size
around the rim of each aperture thereon;
c. depositing needle-forming material substantially perpendicular
to the plane of said plate-like screen member whereby material is
deposited on said surface within said apertures, thereby forming
protuberances within said apertures;
d. depositing a masking material at a shallow grazing angle on said
plate-like screen member thereby to provide a lip of diminishing
size around the rim of each cavity therein at the same time the
said needle-forming material is deposited; and
e. removing all masking material and deposited needle-forming
material from said plate-like screen member.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to field-forming structures such as
field-ionizing and electron-emitting structures and particularly to
such structures employing many cone-like emitters or ionizers on a
single substrate.
Structures for forming electric fields are required for many
practical applications. An electric field on the order of several
megavolts per centimeter (cm) can be used to produce electron
emission from materials. Electric fields on the order of 10.sup.8
to 10.sup.9 volts per centimeter are useful in ionizing molecules
by field extraction and collection of electrons therefrom (known as
field ionization).
Electron emission is of course the heart of devices utilizing
electron beams or clouds such as the many varieties of electron
tubes upon which the electronics industry is built. The phenomenon
of ionization plays a significant role in many scientific
instruments and experiments; e.g. in ionization gauges and mass
spectrometers. In mass spectrometry, an unknown material under
investigation is ionized prior to injection into the analyzer or
mass-separator section of the mass spectrometer. Ionization is
usually produced by electron impact with the unknown material,
utilizing a suitable electron source such as a thermionic emitter.
However, electron impact with molecules not only ionizes them, but
also tends to fragment them into two or more species, so that the
mass spectrum, obtained by this ionization method, may show the
presence of the daughter species but little or nothing of the
parent species. Moreover, if any of the daughter species is the
same as, or has a mass-to-charge ratio approximately equal to,
another species originally present in the unknown material, then
the mass spectrum obtained can be difficult or impossible to
interpret correctly regarding the original constituents of the
unknown material. In some applications where mass spectrometry is
used to monitor or control other processes, e.g. the preparation of
photoemissive surfaces, the use of a thermionic emitter for
ionization is disadvantageous because the heat or light from the
emitter tends to disturb the process. The use of a cold,
non-luminous ionizer in such applications constitutes a significant
improvement. Field ionization, a phenomenon in which molecules
entering a region of very high electric field (10.sup.8 to 10.sup.9
V/cm) are ionized by extraction and collection of electrons by the
field, causes substantially less fragmentation than electronimpact
ionization. Also, this phenomenon does not require or involve the
generation of light or heat.
In order to reduce to a practical level the voltage required for
producing the required high fields, sharp needles or points are
used as emitters or field ionizing electrodes, a counter electrode
is spaced from the needle-like structures and a voltage of
appropriate polarity is applied therebetween. For field emission
the counter electrode is made positive relative to the needle-like
structures and for field ionization the reverse polarities are used
(counterelectrode negative relative to the needle-like structures).
However, even with the use of sharp points, if the counter
electrode is spaced a macroscopic distance from the points, e.g.,
of the order of centimeters (usual in prior art devices), the
voltages required for electron emission are of the order of
kilovolts and for field ionization, approximately tenfold
higher.
Despite the high field emission current density capability of a
single needle-like emitter (on the order of 10 million amps per sq.
cm), the total emission current from a single needle emitter is
low, e.g., on the order of milliamperes, because of the minute size
of its emitting area. Furthermore, the electrons are emitted over a
large solid angle, and they obtain almost the total energy of the
applied voltage, e.g., several thousand electron volts, within a
short distance from the emitter tip. Therefore, the formation of
narrow electron beams that are suitable, for example, for use in
high-power, beam-type electron tubes, requires elaborate and
expensive focusing apparatus.
Ionization efficiency of prior art field ionizers of the single
needle-like structure is very low for reasons similar or analogous
to the problems described above relative to the cathodes. That is,
one reason ionization efficiency is low is that the effective
region where ionization takes place is confined to the small volume
in the immediate vicinity of the apex of the sharp point so that
the rate of ion production for a given pressure of material to be
analyzed is much lower for field ionization than for
electron-impact ionization. A second reason is that the
field-produced ions attain velocities equivalent to the voltage
applied between ionizer and counter electrode and the ions are
impelled away from the ionizer over a very wide range of angles, so
that only a small fraction of the ions are collimated into a beam
suitable for injection into the analyzer of the mass spectrometer
without employing complex ion-optical lenses.
Parallel operation of many needle-like members to increase the
total current for a cathode and to provide a correspondingly large
ionization volume in the case of the field ionizer is feasible, but
the problems of formation of the parallel structures, focusing the
electron beams (for the cathodes), and providing ion-optical
collimation (in the field ionization structures) are formidable.
For example, in the field ionizer case, ion-optical collimation is
practical only if emission energies of the ions can be kept small,
which necessitates spacings between the ionizer and
counter-electrode of the order of microns with the ionizer point
having a tip radius of a fraction of a micron, e.g., 0.1 micron.
Also, it is desirable to sapce the needle-like structure as close
together as possible without incurring significant decrease of the
field at each point by the presence of its neighbors.
Many of the problems thought to be inherent in parallel operation
of fine needle-like structures under consideration have been solved
by a structure and the methods of producing that structure
disclosed in U.S. Pats. Nos. 3,453,478 "Needle-Type Electron
Source", dated July 1, 1969, and 3,497,929, "Method of Making a
Needle-Type Electron Source", dated Mar. 3, 1970 in the names of
Kenneth R. Shoulders and Louis N. Heynick, and assigned to Stanford
Research Institute.
In the patents referred to above, the electric field-producing
structure effectively includes two closely spaced surfaces. On the
first, or emitting surface, a large number of sharp needle-like
emitting sites are distributed with a packing density limited only
by the fabrication technology used. The surface can be planar or
curved and of a size to suit the intended application. The second
surface, called an accelerator surface, is the electrode used to
produce the field. It consists of a very thin foil or film of metal
of the same contour as the surface with the emitter sites, and is
suitably supported and electrically insulated therefrom in spacings
ranging from a fraction of a micron to several microns.
In the preferred embodiment, described in the patents, the
accelerator surface is supported above the emitter surface by a
dielectric layer therebetween, in the manner of a sandwich, and
holes through the accelerator and dielectric layers are provided so
as to expose the tips of several emitters at each hole location to
the rim of the hole in the accelerator electrode. Because of the
minimal separation range between the emitter surface and the
accelerator surface, the voltage needed to produce field emission
ranges from only a few volts to about 100 volts, and the emitted
electrons emerge from the holes in the accelerator with
correspondingly low energies.
While the structure referred to above represents a considerable
advance over any of the structures known to the prior art, the
method of producing the structure can yield needle-like electrodes
that are not necessarily uniform in numbers and shapes from emitter
site to emitter site, thus introducing corresponding variations in
performance. Many of the problems of the multiple-needle structure
are overcome by providing a single, uniform needle-like electrode
at each site with specific, essentially identical, configuration. A
means of producing a single needle-like electrode at each site is
described in an article by C. A. Spindt (one of the inventors of
the present invention) entitled "A ThinFilm Field-Emission Cathode"
in the Journal of Applied Physics, Vol. 39, No. 7, 3,504-3,505,
June 1968. Further, a means of providing a single uniform
needle-like electrode at each site which represents an improvement
over the method and structure described in the previous patents and
the Spindt paper is described and claimed in a U.S. Patent
application Ser. No. 9,139 (now Pat. No. 3,755,704, issued Aug. 28,
1973), entitled "Field Emission Cathode Structure, Devices Using
Such Structure, and Method of Producing Such Structure", filed Feb.
6, 1970, in the names of Louis N. Heynick, Kenneth R. Shoulders,
and Charles A. Spindt and assigned to the assignee of the present
invention.
Subsequent to conception of the structures and methods described in
the above-referenced patents, application and paper, use of similar
structures operated in reverse polarity as a closely spaced
parallel array of field ionizers, in which each sharp metal point
produces positive ions was conceived. In the cathode structure
sandwich, the dielectric film thickness is in the order of 1 -
2.mu. and the metal points are of about the same height above the
emitter surface. However, field ionization in any such structure
having specific values of tip sharpness and distance between
counter electrode and points requires voltages approximately
ten-fold higher between electrodes than those required for field
emission. Consequently, the dielectric layer between the emitter
surface and counter-electrode must be capable of withstanding the
higher fields without dielectric breakdown. This requirement can be
met by making the dielectric thickness large relative to the
distance between the counter-electrode and the tips, or by
providing other means for insuring adequate insulation between the
emitter surface and the counter-electrode.
In addition to providing a multi-point ionizer, the present
invention provides uniform arrays of points, suitable electrode and
counter-electrodes therefor, and improved means for producing such
structures in which the ratio of dielectric thickness to distance
between counter-electrode and tips and also geometrics chosen
optimally for field emitters or ionizers or both.
Particularly in view of the fact that the spacing between emitter
tips and the counter-electrode may be different for field emitters
and field ionizers, it is highly desirable to be able to produce
the fine needle-like points of uniform shape and spacing on a
substrate independent of a metal/dielectric/metal film sandwich.
That is, it is important to be able to produce a precision, highly
uniform bare point array on a substrate (electrode most commonly).
With such a structure, one or more counter-electrodes may be added
with the desired spacing, dielectric thickness or other adequate
insulation, and the proper registry relative to the points of the
bare point array. The present invention provides the capability of
producing such results.
As described in greater detail below, in accordance with the
teachings of the present invention a bare-point structure is
provided in which a regular array of closely spaced metallic points
of controlled geometry is formed by deposition through a fine mesh
plate or screen uniformly over the surface of a metal substrate
which represents an electrode.
Where the bare-point array is desired, the screen may be removed.
Where a counter-electrode is desired, the screen may be left in
place or removed and replaced by another counter-electrode of
desired configuration. A field ionization structure is provided by
making the counterelectrode of the arrangement just described
negative relative to the substrate electrode and providing the
proper electrode-counter-electrode spacing as well as ratio of such
spacing to the distance between counter-electrode and electrode
points. The field emitter is provided by applying the opposite
polarity between electrodes and providing optimally different
spacings. Additional electrodes can be added to the structure to
provide multi-electrode control of the electron or ion optical
characteristics as well as the current emerging from the holes.
Multi-element vacuum tubes can also be produced by adding
appropriate electrodes and closing the device. Further, the field
ionizer may be constructed by the same general method described in
connection with the Heynick, Shoulders, and Spindt application
referred to above with modifications described herein.
The novel features which are believed to be characteristic of the
invention are set forth with particularity in the appended claims.
The invention itself, however, both as to its organization and
method of operation, together with further objectives and
advantages thereof, may best be understood by reference to the
following description taken in connection with the accompanying
drawings in which:
FIG. 1 is an enlarged fragmentary perspective view, showing a
bare-point array (pyramidal embodiment) constructed in accordance
with the principles of the present invention;
FIG. 2 is an enlarged fragmentary perspective view of a portion of
a device utilizing the bare-point array of FIG. 1 and constructed
in accordance with this invention;
FIGS. 3 through 5, inclusive, are cross-sectional views taken along
the lines 3--3 of FIG. 2 for successive steps in the method of
producing the structure of FIGS. 1 and 2;
FIG. 6 is a cross-sectional view similar to the device of FIG. 5,
but illustrating another embodiment which is constructed in a
different way;
FIG. 7 is an enlarged fragmentary perspective view of a field
ionizer according to one embodiment of the present invention;
and
FIG. 8 is a broken-away cross-sectional along lines 8--8 in FIG.
7.
FIG. 9 is a partially broken-away cross-sectional view of another
embodiment of the invention.
A form of the basic bare-point array 10 useful for both field
electron emitters and field ionization is illustrated in FIG. 1.
The structure 10 includes a substrate 11 and an array of bare
points 12 formed thereon. In the embodiment shown, the bare points
12 are pyramidal but may be of other conical shapes. The substrate
11 is preferably conductive in order to form one electrode. In the
embodiment illustrated, substrate 11 is a sheet of molybdenum but
it may be of other suitable metal, or a non-metal coated with a
conductive film as, for example, a plate of aluminum oxide coated
with a film of molybdenum. For some applications, it may be
preferable to use a semi-conductive material or even an insulator
for the substrate 11. As illustrated, the pyramids 12 are of
molybdenum, have square bases, are 0.6 mil high, and are spaced
apart by 1 mil (center to center). However, the pyramids 12 may be
of resistive or insulating materials, or of composite materials,
and the pyramid surfaces overcoated or otherwise treated to obtain
the desired characteristics.
Bare-point arrays 10 require a field-producing electrode in order
to produce the electric field required to cause electron emission
or ionization in the region of the array of points or pyramids 12.
The electrode is preferably but not necessarily analogous to the
top conduction film in the sandwich configurations described in the
previously cited patents and application. FIG. 2 illustrates a
device incorporating the bare-point array of FIG. 1 and the
additional electrode 12 (referred to as a counter-electrode) to
provide the required electric field. As illustrated, the
counter-electrode 13 comprises a screen (or plate) having a
distribution of holes or apertures 14 therein, shown square in this
embodiment, corresponding substantially to the distribution of
points 12 on the substrate. The broken-away cross-sectional view of
FIG. 5 may help to visualize the device. Looking at the two Figures
(2 and 5) it is seen that the device comprises a substrate 11
having points 12 formed thereon and a screen (counter-electrode 13)
supported above the substrate by an insulating spacer 15 at the
periphery of the screen. In a preferred version of this embodiment,
registration between the screen 13 and the substrate 11 is
maintained by spacer 15 so that the center of each screen hole 14
is substantially aligned with the axis of a different point 12 of
the base-point array 10, and so that the tips of the points 12 are
substantially in the plane of the screen 13. Furthermore, the ratio
of the height of points 12 above substrate 11 to the distance
between the tips of the points 12 and the rims of the screen holes
14 is sufficiently large so that the requisite voltages for
electron emission or field ionization may be applied between
substrate 11 and counter-electrode 13 without causing electrical
breakdown of insulator spacer 15. One means of assuring that the
breakdown doesn't occur is seen in another version of this
embodiment which is illustrated in FIG. 9.
For convenience and simplicity, the illustration of FIG. 9 has
parts which correspond to those of FIGS. 2 through 6, inclusive,
numbered correspondingly. Here, the perimeter of the substrate 11
(or counter-electrode 13, if preferred) is shaped so as to permit
the use of a thicker insulator spacer 15, thereby permitting the
application of higher voltages between substrate 11 and
counter-electrode 13 without causing electrical breakdown of
insulator spacer 15.
The basic mode of operation of the field ionizer may be best
explained in connection with FIGS. 2 and 5 wherein the ionizer
points 12 of array 10 are shown connected by means of their common
conductive substrate 11 to a positive (+) terminal of a voltage
power supply 16. The sharp tip points 12 are each located in or
near a hole 14 of counter-electrode (screen) 13, which is connected
to the negative (-) terminal of the power supply 16. Application of
a voltage from power supply 16 produces a high electric field in
the region of the points 12. In such an arrangement, electrically
neutral particles entering the holes 14 are positively ionized by
the high electric field, the action of the field being to remove
electrons from the particles, which electrons are collected by the
points 12. The positive ions so created are impelled away from the
ionizer points 12 through the holes 14 of the counter-electrode
13.
For producing field electron emission, the potential source is
connected with its positive terminal to counter-electrode
(accelerator electrode) 13, and its negative terminal connected to
array (emitter electrode) 10. The potential source may be made
variable for the purpose of controlling the electron emission
current. Upon application of a potential between the electrodes 10
and 13 an electric field is established between the points
(emitting protuberances) 12 and the counter-electrode 13, which is
of a polarity to cause electrons to be emitted from the points 12
through the holes 14 in the screen 13.
Thus it is seen that efficiency limitations of one ionizer point or
one field emitter cathode point as well as the limitations of prior
attempts at parallel operation of such single point devices are
largely overcome by providing a structure consisting of an array 10
of closely spaced points 12 with sharp tips in close proximity to
the counter-electrode 13 which has a corresponding array of holes
14. In this structure the holes 14, the distance between points 12
or holes 14, as well as the spacing between point tips and the
counter-electrode, are in the micron range, and most points 12
yield substantially equivalent performance. Insulator spacers 15
which separate the outer edges of the electrode 10 and
counter-electrode 13 have the requisite thickness to withstand the
electric field.
The ability to produce the bare-point array 10 with such uniformity
of points 12 and hence the ability to produce field ionizers and
field electron emitters of such precision and efficiency is highly
dependent upon the methods of construction. The method of the
present invention yields the precise results desired.
In order to understand the steps in one method used in the
fabrication of the array 10 (of FIG. 1) and the completed device
(FIGS. 2 and 5), reference may be had specifically to FIGS. 3
through 5, inclusive, which represent sections through one portion
of the device illustrated in FIG. 2. FIG. 3 illustrates the
substrate 11 of the bare-point array structure 10 before the
field-forming points 12 are formed thereon. That is, FIG. 3 shows a
starter structure consisting of only the substrate 11 and a fine
mesh screen (plate) 13 having a multiplicity of holes or apertures
14 therein supported above the substrate 11 by an insulating
dielectric spacer 15. If the screen 13 is later to be removed to
provide only the bare-point array 10 (of FIG. 1), the screen may be
placed in direct contact with substrate 11 and the dielectric
spacer 15 may be eliminated. Since this embodiment contemplates
that the masking screen and the counter-electrode 13 will be one
and the same, the the spacer 15 is shown, and also a release layer
18 is provided on the screen 13 so that materials subsequently
deposited thereon in the array-forming process may readily be
removed.
In order to provide the sharp points 12 as shown in FIG. 4 and
thereby complete the pyramidal array 10 of FIG. 1, a simultaneous
deposition from two sources is performed. That is, simultaneously a
closure material (e.g., a molybdenum-alumina composite) is
deposited at a grazing incidence, and the material for the pyramid,
e.g. molybdenum, is deposited straight on the substrate surface. In
this step, the purpose of the deposition at grazing incidence is to
add material on screen 13 so as to provide a mask with holes 14 of
decreasing size for the deposition of material on substrate 11. As
additional emitter material is deposited on substrate 11, the
molybdenum-alumina composite masking material gradually closes the
aperture at the upper lip of the holes 14, as shown in FIG. 4. The
closure is indicated by the additional film 19 deposited on the
release layer 18. In FIG. 4 the apertures 14 are shown as being
completely closed and pyramids 12 completely formed. Thus,
cone-shaped (pyramidal here) points are formed on the conductive
substrate 11. If the screen 13 used is provided with round
apertures instead of the square ones shown, then the points 12
formed are right circular cones instead of the pyramids
illustrated.
With the step just described, the array 10 of FIG. 1 is completed
and the screen 13 and spacer 15 may be removed, leaving the
bare-point array 10. Another screen-like counter-electrode can then
be added to form the structure of FIG. 2. If it is desired to use
the screen 13 as counter-electrode of FIG. 2, screen 13 and spacer
15 need not be removed. Instead, the materials deposited on the
screen, viz., release layer 18 and the subsequently deposited
closure layer 19 may be selectively etched or floated away, leaving
the bare screen structure as illustrated in FIGS. 5 and 2.
If larger spacings between counter-electrode 13 and substrate 11
are desired, to accommodate thicker dielectric spacers 15, the
points 12 may be formed on previously produced pedestals (not
shown), which pedestals are produced by a prior deposition step,
utilizing a source which deposits material along a direction
perpendicular to the surface of the substrate, and which material
is preferably the same material, e.g. molybdenum, as the metal
electrode (substrate) 11 or a more resistive material, e.g. a
molybdenum-alumina composition. Such a deposition step would
deposit a film on the release layer 18 without closing the screen
holes 14 and, more importantly, pedestals with essentially vertical
sides and bases of size and shape of apertures 14 in the screen 13
are deposited directly upon the substrate 11. The pedestal height
is selected by controlling the amount of material deposited. Since
the array of FIG. 1 does not have such pedestals, this step is not
illustrated. However, a specific embodiment of a complete device
incorporating such pedestals is shown in FIGS. 7 and 8, described
later herein.
In the embodiment illustrated, the screen 13 has a uniform array of
square holes 14, spaced on 1 mil centers. The pyramids formed then
have square bases of corresponding size and spacing to the screen
mesh and of a height controlled by the relative rates of deposition
of the sources. In other embodiments, the holes 14 may have other
configurations and/or the deposition rates may be varied during the
formation process to provide a variety of shapes. Further, the
formation process may be halted prior to hole closure so as to form
truncated pyramids, cones, or suitable variants thereof.
An alternative deposition technique incorporates the use of a
single deposition source which is broad enough to perform both the
hole-closure and point-formation functions. This deposition source
and technique may also be applied to the sandwich starter structure
described in the previously cited patents and applications.
One embodiment specifically designed for field ionizer application
and utilizing a metal/dielectric/metal sandwich structure with a
counter-electrode 30 which corresponds to the screen
counter-electrode 13 of FIG. 2 is illustrated in FIGS. 7 and 8. In
this embodiment the counter-electrode 30 is formed of the upper
metal film of the sandwich structure which is provided with a
plurality of holes or apertures 28 therethrough. Dielectric film
31, the center layer of the sandwich, is on top of a base metal
film 32 which serves as a base electrode and which is connected to
the positive terminal of the power supply 24. As illustrated, the
base metal film 32 is shown on a dielectric support substrate 43
which only serves to support the base metal film 32. Films 30 and
32 are formed of metal such as molybdenum or tungsten, while film
31 which insulates films 30 and 32 from one another is formed of a
dielectric material, e.g., aluminum oxide.
The dielectric film 31 has holes corresponding to the holes in film
30 and each hole accommodates a point ionizer 40 with its base in
contact with the base electrode 32 and tip 26 preferably aligned
with the plane of the hole 28 in the top electrode 30 to minimize
the distance between tip and hole rim. FIG. 8 is a cross-sectional
view along lines 8--8 in FIG. 7. In FIG. 8 the hole in dielectric
film 31 is designated by numeral 36. The point ionizer 40 is shown
in the form of a cone 41 on top of a pedestal 42, a configuration
that permits independent selection of the height of the tips 26
above base 32, the sharpness of the tip 26, and the distance
between tips 26 and hole rims 28 so as to provide optimum geometry
for operation of the structure as a field ionizer.
Another highly practical way to utilize the bare-point arrays 10 is
shown cross-sectionally in FIG. 6. In this embodiment a conductive
screen 13 having substantially the same distribution of holes as
the points 12 on the substrate 11 is supported above the substrate
11 by insulator spacers 23 of appropriate height and distribution.
One method for producing such structures is to form an insulator 23
of the requisite thickness on the screen 13 by deposition or other
means, which insulator thereby conforms substantially to the
cellular structure of the screen, after which the screen-insulator
combination is set and maintained on the substrate. An advantage of
this method is that self-registry of holes and points is
achieved.
Bare arrays of points can be made to yield very large emission
currents by the use of an electrode to which appropriate positive
potentials relative to the points are applied, e.g., in diode
rectifiers, x-ray generating tubes, and Lenard-ray tubes.
Therefore, it is contemplated that bare-point arrays 10 or
individual members of such arrays be sealed off opposed to another
electrode either with or without intermediate electrodes.
Substrate-screen assemblies having the points in substantial or
complete registration with the holes in the screen can be used as
large-emission current cathodes by applying suitable positive
potentials to the screen relative to the substrate. In
contradistinction to the operation of the diode configurations
cited above, the screen provides the fields required for electron
emission for the points, but most of the emission drawn passes
through the screen holes, so that the screen functions to control
the current in the manner of a grid. Additional grids may also be
employed to render the emission more uniform or otherwise control
the emission.
The methods for producing an array of points in registry with holes
in screens are adaptable to the production of cathodes subdivided
into areas containing one or more points, from which areas emission
can be drawn separately by the application of appropriate
potentials thereto. Such methods can also be adapted to the
production of arrays of individual but suitably interconnected
field emission diodes, triodes, tetrodes, etc.
All the operational advantages of the multipoint field ionizer
described above are achieved also with the field emitters of the
same general structure with parameters optimized for such use.
Again, such structures constitute field ionizers when operated with
reverse polarities to those used for obtaining field emitted
electrons, and such structures can be produced by the methods
previously described so that the values of the geometric parameters
are optimum for field ionization use. This is not to say, however,
that the method of producing the ionizer from the
metal/dielectric/metal sandwich is equivalent to, or can be made
with, the same degree of accuracy as the improved screen-forming
techniques herein described.
While particular embodiments of the invention are shown, it will be
understood that the invention is not limited to these structures
since many modifications may be made both in the material and
arrangement of elements. It is contemplated that the appended
claims will cover such modifications as fall within the true spirit
and scope of this invention.
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